Prior and current work has shown that transduction of a genetic determinant of methicillin-resistance in Staphylococcus aureus is dependent on the presence of an appropriate prophage and, in most cases, an appropriate penicillinase plasmid in the recipient cell. Elimination of the prophage from a methicillin- resistant transductant reduced resistance to undetectable levels in 6 of 7 eliminations. In two of these, no trace of resistance could be restored even after the prophage was re-introduced. In future work, we plan to prepare mutants of penicillinase plasmids and prophages and test them for ability to make staphylococci effective transductional recipients of methicillin resistance.